In the drilling of oil and gas wells, downhole drilling motors may be connected to a drill string to rotate and steer a drill bit. Conventional drilling motors typically include a power assembly, a transmission assembly, and a bearing assembly. Rotation is provided by the power assembly that may be a positive displacement motor driven by drilling fluid (mud) circulation. The transmission assembly transmits torque and speed from the power assembly to a drill bit disposed at a lower end of the drilling motor. The bearing assembly takes up the axial and radial loads imparted on the drill string during drilling.
For example, conventional drilling motor 2 shown in FIGS. 1A and 1B includes power assembly 4, transmission assembly 6, and bearing assembly 8. Power assembly 4 may include stator housing 10, stator 12 secured within stator housing 10, and rotor 14 rotatable within stator 12. Stator 12 may have a helically-contoured inner surface. Rotor 14 may have a helically-contoured outer surface. Together, they define a positive displacement fluid motor having a helically-shaped progressive cavity. Bearing assembly 8 may include bearing housing 16 and rotatable drive shaft 18 secured within bearing housing 16. Transmission assembly 6 may include transmission housing 20 and transmission shaft assembly 22 for connecting rotor 14 to drive shaft 18.
Drilling fluid or mud is pumped from the well surface through the drill string or drill pipe to drilling motor 2. The drilling fluid or mud flows through the cavity formed between rotor 14 and stator 12, through transmission housing 20 around transmission shaft assembly 22, into the inlet ports provided on drive shaft 18 or a portion of transmission shaft assembly 22, through drive shaft 18, and out through the outlet port to flush cuttings from the wellbore and cool the drill bit. The drilling fluid drives rotor 14 to rotate within stator 12, with rotor 14 orbiting around the inner surface of stator 12. Transmission shaft assembly 22 transmits the rotational movements of rotor 14 to drive shaft 18. Drive shaft 18 rotates concentrically within bearing housing 16 to drive the drill bit.
Transmission shaft assembly 22 may be formed of a constant-velocity joint (“CV joint”). The CV joint may include rotor adapter 24 coupled to rotor 14, drive shaft adapter 26 coupled to drive shaft 18, and center shaft 28 coupling rotor adapter 24 to drive shaft adapter 26. The CV joint may further include thrust members 30 and shear members 32 at the upper and lower ends of center shaft 28. Shear members 32 may contact an outer surface of center shaft 28 and an inner surface of rotor adapter 24 and drive shaft adapter 26. Through continuous contact, shear members 32 cause wear on the inner surface of rotor adapter 24 and drive shaft adapter 26, which may lead to failure of rotor adapter 24 and drive shaft adapter 26.